Various reversible image forming materials have hitherto been disclosed in many references including Unexamined Published Japanese Patent Application Nos. 191190/1983, 193691/1985, U.S. Pat. No. 3,666,525, Unexamined Published Japanese Patent Application Nos. 119377/1979, 39377/1988, 41186/1988, U.S. Pat. No. 4,028,118, and Unexamined Published Japanese Patent Application Nos. 81157/1975 and 105555/1975.
Among the methods disclosed in these references, those which are disclosed in U.S. Pat. No. 4,028,118, as well as Unexamined Published Japanese Patent Application Nos. 81157/1975 and 105555/1975 have had the disadvantage that the image formed will vary with temperature and hence is unsatisfactory in terms of memory quality. The image forming materials described in Unexamined Published Japanese Patent Application Nos. 191190/1983, 193691/1985, and U.S. Pat. No. 3,666,525 comprise a recording layer composed of a color former, a color developer and a binder, with a low vapor-pressure solvent or a heat-fusible material added as required. In those image-forming materials, color is formed using thermal energy and erased with the aid of water, water vapor or a certain kind of organic solvent and, hence, the mechanism involved differs from the one which accomplishes cyclic color formation and erasure solely by controlling thermal energy.
The image forming materials described in Unexamined Published Japanese Patent Application Nos. 119377/1979, 39377/1988 and 41186/1988 have a heat-sensitive layer that is chiefly composed of a resin matrix and an organic low-molecular weight material dispersed in said resin matrix. The recording method they adopt depends on the control of thermal energy, which causes reversible changes in the transparency of the heat-sensitive layer to form and erase images. This is not a method of forming and erasing images by chemical color formation and erasure.
As described above, there has been no prior art system that is capable of chemical color formation and erasure solely by controlling thermal energy and which has already been practiced commercially.
An object, therefore, of the present invention is to provide a reversible thermal recording medium that has not been proposed in the prior art and which has memory quality in that it is capable of chemically forming or erasing color solely by controlling thermal energy, as well as a composition suitable for use in that medium.